The present invention relates to the art of digital imaging. It finds particular application in macro uniformity corrections for x-y separable non-uniformities in a raster output scanning (ROS) printing system and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Macro non-uniformity levels have existed in raster scan image output terminals (IOTs) (e.g., xerographic printers) for some time and are a concern for most marking processes. Even small non-uniformity level errors in raster scan IOTs give rise to visually objectionable banding in halftone outputs (e.g., image macro non-uniformity streak artifacts). Such errors typically arise in raster scan image output terminals (IOTs) due to variations in ROS spot size across the field (which is constant in time (print to print)), donor-roll once-around, HSD wire hysteresis, laser diode variations, LED bar power variation, ROS scan line non-uniformity, photoreceptor belt sensitivity variations, and/or ROS velocity non-uniformity. Significantly, many variations occur only in the fast scan (e.g., X) or slow scan (e.g., Y) directions, and they do not interact to first order. Therefore, a correction made in one direction has a negligible effect on artifacts in the other direction. Other printing technologies (e.g. thermal inkjet and acoustical ink printing) also have artifacts that occur in a regular, predictable manner in one or both directions and fall within the scope of this discussion.
Although techniques have been proposed to eliminate such non-uniformity errors by making physical systems more uniform, it is too expensive to control or limit the error to an acceptable level, below which the error will not be detected by the unaided eye. Fixes have been attempted in the marking process, but not enough latitude exists to fully solve the problem. For problem sources such as LED non-uniformity, the correction is sometimes addressed with current control or pulse width control. However, none of the solutions discussed above implements a technique based in digital electronics. With the cost of computing rapidly decreasing, such digital electronics based solutions are becoming more attractive.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.
A method for rendering a raster output level determines an image position of a pixel of interest (POI) within an image. An intended raster output level, which corresponds to the POI, is received into a processing device. A final raster input level is determined as a function of the image position and the intended raster output level. The final raster input level and the image position are transmitted to an output device. An actual raster output level is rendered, via the output device, at a position on an output medium corresponding to the image position. The actual raster output level substantially matches the intended raster output level.
In accordance with one embodiment of the invention, a plurality of correction curves is computed for respective raster output levels. One of the correction curves is identified as a master correction curve. A scaling function is determined in accordance with relationships between the master correction curve and the other correction curves. The scaling function is used for producing the final raster input level.
In accordance with a more limited aspect of the invention, averages of actual output levels, which are produced by the output device for the raster output level of the master correction curve, are determined over a non-correctable direction at respective positions along a correctable direction of the output device. The correctable and non-correctable directions are substantially perpendicular. The relationships between the master correction curve and the other correction curves are determined as a function of the averages of the actual output levels.
In accordance with a more limited aspect of the invention, a plurality of tone reproduction curves is calibrated for one of the correction curves.
In accordance with a more limited aspect of the invention, the calibrating step includes, for each of the positions along the correctable direction, storing an identifier of the respective tone reproduction curve, which most closely achieves the final output level as a function of the respective position, in a lookup table.
In accordance with another aspect of the invention, the actual raster output level is printed.
In accordance with a more limited aspect of the invention, the actual raster output level is printed on a xerographic color printing device.
One advantage of the present invention is that it may reduce the number of tone reproduction curves necessary for correcting macro non-uniformities (as compared to a case where different tone reproduction curves are applied for each row or column of pixels or a case if one tone reproduction curve is stored uniquely for each pixel).
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.